In a conventional lithographic device, a light source emits a light beam that passes through an optical projection device fitted with the mask so as to project an image of this mask onto the chip. The characteristics of the image projected, notably the resolution and field depth, are chiefly linked to the exposure wavelengths and the numerical aperture of the objectives used in the optical projection device. The trend to miniaturise integrated circuits involves improving these characteristics.
One method of improvement is to use ever shorter emission wavelengths. Thus, it has been possible to progress from the use of high pressure mercury vapour lamps operating at wavelengths of about 405 nm, then 365 nm, to the use of excimer lasers operating at wavelengths of 248 nm and then 193 nm, resulting in a substantial reduction in the emission wavelengths. Going any lower, so as to achieve a wavelength of 157 nm, for example, creates problems, especially in the production of optical projection devices.
Another, additional, method of improvement sets out to increase the numerical aperture of the optical devices.
Conventionally, the propagation medium that separates the optical projection device from the lithographic device of the silicon chips is air, in which the values of the numerical aperture are limited to 1. It is possible to increase the numerical aperture still further by replacing air with a liquid having an index greater than 1, for example water or any other suitable immersion liquid. This is known as “immersion photolithography”.
However, the theoretical performance of immersion photolithography is far from being achieved in practice because, at this level, there are significant variations in the properties of the optical projection device, with its immersion liquid placed between the optical projection device and the silicon chip. Thus, the quality of the image projected and/or its magnification change as a function of the different operating parameters of the lithographic device, particularly the refractive index of the liquid, the emission wavelength of the light source, the temperature of the optical projection device, ambient temperature, and the temperature of the chip.
It is desirable to be able to control the variations in these parameters, as any variation of this type causes a degradation of the integrated circuits manufactured and/or a loss of yield which rapidly becomes unacceptable. For example, the working wavelength of the light source can be controlled with a precision of the order of 5.10−7; it is possible to control the temperature of the optical projection device and ambient temperature with a precision of the order of 0.005° C.
The refractive index of the liquid still remains. Variations in this index influence the quality (resolution) of the projected image and/or its magnification. The variations in the refractive index are linked to various reasons, for example the purity of the liquid, the emission wavelengths, the temperature of the liquid and the pressure of the liquid. These variations ought to be controlled with a precision comparable to that of the wavelength of the laser of the light source.
Existing equipment and methods of monitoring are still not entirely satisfactory, even the best (“Immersion Lithography Workshop”, 27 Jan. 2004, published by the NIST, National Institute of Standards and Technology).
The invention sets out to improve the situation.